PSI - Issue 47

Mario A. Sánchez Miranda et al. / Procedia Structural Integrity 47 (2023) 310–324 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Hardness values in the stir zone decreases in the FSW process, as the tool-pin temperature rises; such tendency has been observed in investigations related to FSW of polymers [37,43]. At the origin of this behaviour is the increase on heat dissipation with the increase of the rotating speed, which lead to softening the materials at the stir zone with the consequently hardness reduction. In the PP and HMWPE zones, hardness VH correspond to parent materials: close to 5 VH for PP and 3.5 – 4 VH for HMWPE. In this work, a stationary shoulder to improve the mechanical properties of HMWPE- PP dissimilar joints have been described, together with the determination of process parameters inducing the higher tensile strength on these joint. Furthermore, the analysis of laminar to turbulent flow transition and from brittle to ductile fracture behaviour have been undertaken, and the microhardness HV has been measured and discussed in terms of heat dissipation and crystallinity.

Conclusions The following conclusions can be drawn from the present work:

• Friction stir welding has been performed in dissimilar thermoplastics: HMWPE and PP, with and without preheating. • Best results of joining were obtained for specimens without preheating and at lower values of rotating and transverse speeds. At origin of this behaviour may be the low glass transition temperature of these two thermoplastics, which lead to adequate molecular diffusion in liquid state and mixing of the polymeric chains, without need of preheating. • Tunnel effect and micro porosities were not observed for specimen destined to tensile tests; the decrease on tensile strength for dissimilar joining in Figures 8b, 8c and 8d, may be related to excessive heat generation during the welding process. • All tensile tests of Figure 8 show an instantaneous fracture, without a period of stress decreasing with elongation increasing, as in the parent materials. The brittle fracture was predominated in welded specimens, over the ductile fracture. • Transition from laminar to turbulent flow leads to decreasing in tensile strength; nevertheless, brittle fracture was observed for all tested dissimilar HMWPE-PP joints. • Microhardness values were obtained along the stir zone and neighbourhood. In the stir zone these values decrease, as a consequence of material softening induced by heat generation and low heat dissipation in thermoplastics with low heat conducting properties. Hardness values outside the stir zone, were close to those of parent materials for the two thermoplastics. • It seems necessary to investigate additional parameters for the process of joining these dissimilar thermoplastics under FSW technique, particularly: the relationship between the process parameters and the microscopic structure in terms of : mixing and distribution of polymeric chains of both thermoplastics after FSW [44], the presence of micro porosities or discontinuities, the effect of initial temperature on the mechanical properties, the transition point from brittle to ductile fracture, the transition from laminar to turbulent flow, …

Acknowledgements

The authors express their mention of gratitude to CONACYT (The National Council for Science and Technology, Mexico), for the financial support destined to this work through the program grant: CB-241117-2014.